The softness has been annoying the crud out of me also. Being a newbie, how can I eliminate/reduce diffraction? Is there a way to head it off or just something I have to deal with post-process? (I'm searching the boards on the topic now too).

I was just reading that the larger aperature the more diffraction there is, so stopping it down would produce less. DCains - it sounds like it's more of an inverted bell curve idea (diffraction on the vertical and f-stop on the horizontal) than a linear progression. Is that correct?

The smaller the aperture (larger f/stop number), the greater the diffraction. I can't say whether the progression is linear, or not, but the higher the pixel density, the bigger risk your camera sensor has of creating diffraction.

As for aperture, diffraction, and DoF, it will all be a compromise, so play around and see what the limits are of each body/lens combination you have. I can use a much smaller aperture with my 10D (6 megapixels) than I can with my 1Ds2 (17 megapixels), but that's the way it is. You should check out a DoF calculator (several free online). Also, google "hyperfocal distance", which is useful for landscape photography.